As electrolytes for currently commercially available non-aqueous electrolyte batteries such as lithium primary batteries, lithium secondary batteries and the like, organic electrolytes formed of electrolytic salts dissolved in organic solvents are commonly used. However, the organic electrolytes are liable to leak out of components, generate the elution of electrode materials and volatilize. Therefore, there have been a problem in long-term reliability and a problem of scattering electrolytes during a sealing process.
Recently, instead of using metallic lithium or its alloy for negative electrodes, have been developed carbon materials utilizing absorption-desorption of lithium ions and matrix materials such as electroconductive polymers. Thereby, it has become possible in principle to avoid generation of dendrite, which takes place with the case where metallic lithium or its alloy is used. Consequently, the incidence of short circuit within batteries has dropped sharply. The carbon materials, especially, are known to have a lithium absorption-desorption potential close to a lithium deposition-dissolution potential. Among the carbon materials, graphite has a large capacity per unit weight and unit volume since it can take lithium atoms within its crystal lattice in a proportion of one lithium atom to six carbon atoms theoretically. Furthermore, graphite provides a flat lithium intercalation-deintercalation potential and is chemically stable. Graphite can contribute greatly to the cycle stability of batteries.
For example, J. Electronchm. Soc., Vol. 137, 2009 (1990), Japanese Unexamined Patent Publications Nos. HEI 4 (1992)-115457, HEI 4 (1992)-115458 and HEI 4 (1992)-237971 disclose batteries using graphite type carbon materials as negative electrode active materials, and Japanese Unexamined Patent Publications Nos. HEI 4 (1992)-368778, HEI 5 (1993)-28996 and HEI 5 (1993)-114421 disclose batteries using surface-treated graphite type carbon materials as negative electrode active materials.
As described above, the graphite type carbon materials can provide a discharge capacity almost equal to a theoretical capacity in organic electrolytes formed mainly of ethylene carbonate (EC). Since the charge-discharge potential thereof is slightly higher than the lithium dissolution-deposition potential and is extremely flat, it is possible to realize high-capacity secondary batteries with flat battery voltage by producing the batteries using graphite type carbon materials as negative electrode active materials.
Thus the capacity of batteries can be raised with the graphite type carbon materials, but there still remains a problem in that the graphite type carbon materials cause decomposition of organic electrolytes due to their high crystallinity. For example, propylene carbonate (PC), which is a solvent for organic electrolytes, is widely used as a solvent for electrolytes of lithium batteries since it has a large potential window, a low coagulation point (−70° C.) and a high chemical stability.
However, it is reported in J. Electrochm. Soc., Vol. 142, 1746 (1995) that, in the case where a graphite type carbon material is used as a negative electrode active material, PC decomposes significantly and the electrode formed of the graphite material cannot be charged or discharged if only 10% of PC is present in the electrolyte.
In recent years, reports have been made about organic electrolytes of EC mixed with various low-viscosity solvents for improving ion conductivity at low temperatures. However, there remain problems in volatility and leakage of such organic electrolytes.
For the purpose of improving the leakage-proof property, safety and long-term storability, ion-conductive polymers having a high ion conductivity have been reported, and are extensively studied as one means for solving the above-mentioned problems. As one type of ion-conductive polymers presently under study, homopolymers and copolymers composed of ethylene oxide as a fundamental unit, which are in the form of straight-chain polymers, crosslinked network polymers or comb-form polymers have been proposed and are almost put in practical use. Various batteries using the above-mentioned ion-conductive polymers are described in patent publications and others, which are typified, for example, by U.S. Pat. No. 4,303,784 (1981) to Armand et. al., U.S. Pat. No. 4,589,197 (1986) to North and U.S. Pat. No. 4,547,440 (1985) to Hooper et. al. These disclosures are characterized by using ion-conductive polymers wherein electrolytic salts are dissolved in polymeric materials having polyether structure. These proposed ion-conductive polymers are under research and development as electrolytes for large-size lithium batteries to be power sources of electric automobiles. However, since the ion-conductive polymers described above have low ion conductivity at temperatures below room temperature, it is difficult to realize small-size, light-weight batteries with high energy density which are demanded for power sources for driving portable electronic instruments and for memory back-up.
On the other hand, as means for further improving the ion conductivity more than the above-described ion-conductive polymers improve it, are proposed methods of adding organic solvents (especially preferably organic solvents with high dielectric constant such as EC or PC) to ion-conductive polymers while maintaining a solid state, as typified by Japanese Unexamined Patent Publication Nos. SHO 59 (1984)-149601 and SHO 58 (1983)-75779 and U.S. Pat. No. 4,792,504. However, in the cases where these proposed methods are used, the ion conductivity is surely improved, but film strength declines significantly. In other words, even in the cases where these proposed methods are used, there is also a possibility that electrolyte layers are deformed and destroyed by compression and slight short circuits take place after batteries or electrochromic devices are actually assembled by inserting thin films of the ion-conductive polymers between electrodes.
Further, in secondary batteries, as the volume of electrode active materials expands and contracts at charging and discharging, the electrolyte layers also receive compression and relaxation stress. Accordingly, it is also necessary to consider not only the improvement of the ion conductivity but also the improvement of mechanical properties for improving the performance of the ion-conductive polymers.